The present invention relates generally to the manufacture of sharpening stones. More particularly, the present invention relates to a process for artificially producing a high quality, high density abrasive novaculite product capable for use as a sharpening stone or the like.
As will be recognized by those skilled in the art, novaculite comprises a form of quartz which itself is primarily comprised of silicon dioxide. The mineral quartz is an oxide of the non metallic element silicon and it appears in a greater number of forms than any other mineral. Quartz is an essential constituent of many igneous rocks such as granite. In metamorphic rocks quartz constituent figures vary largely in the gneisses and in quarzite.
In its natural form novaculite has been mined, particularly in Arkansas, for use as a sharpening stone. It is increasingly becoming rate. Naturally occurring novaculite typically exhibits density of approximately 2.60 in molecular form, 2.3 in stone form and a MOH hardness of between 7-7.5, with natural stone of 7.5 MOH hardness being very rare. At the present rate of consumption in domestic cutting shops it is likely that presently known deposits of high grade novaculite will be consumed in the next thirty years. One reason for this is that conventional cutting processes inevitably waste approximately 75% of all quarried stone, in order to arrive at an end product stone which lacks flaws and which is suitable for honing. The rest will become seconds, or scrap. Already the translucent variety and Washita type are rare and very expensive whereas ten years ago these varieties constituted the bulk of Novaculite volume. There is now only one known commercial grade Washita deposit, and it is believed that it will expire within the next five to ten years.
A typical sharpening stone cut from naturally occurring, mined novaculite suffers from several imperfections. First, naturally occurring novaculite is not homogenous and regions of different density exist in the stone. Fissures such as quartz intrusions, cleavage or the like exist along certain fault lines which weaken the stone, and which indicate a region of differential density or sharpening characteristics. Natural stone may experience such deleterious fissure through natural processes when subject to freezing during the winter, and as a result most quarries must mine suitable novaculite at a substantial distance beneath the ground. This further aggravates the present shortage of high grade Novaculite. Other imperfections include sand pits, fault lines and quartz seams.
Naturally occurring novaculite stone suitable for cutting into sharpening stone also suffers from a plurality of various contaminating incursions. These are minor imperfections in the surface or the like such as sand pits which are formed when the stone is cut through and portions of sand naturally formed within the item being cut are exposed. Such sand pits continue to widen and enlarge as time goes on so that such a conventional finished stone product, when used as a sharpener, degrades.
Natural novaculite also exhibits substantial colour and density differences which effect the aesthetic value of the unit. Naturally occurring novaculite subject to the aforementioned difficulties may further interfere with normal knife sharpening. For example, if the stone consists of two substantially different density regions the softer or lower density region, characterized by a high coefficient of friction, will exhibit a grinding effect upon the knife being sharpened. However, the adjacent harder or higher density region will exhibit less friction, so as to be suited more for polishing the knife. Generally the uniformity of color of sample novaculite correlates to the uniformity of its density. Serious knife makers and knife sharpeners prefer a uniform density so that high standards of quality control can be maintained in the final knife blade or edge "polishing" stages of production.
Thus it would seem desirable to provide a system for artificially manufacturing novaculite exhibiting uniform characteristics, which does not include contaminant incursions, and which exhibits substantially homogenous density so as to consistently either "polish" or "grind" as desired by the user.
Moreover, it is desirable to produce such a stone with uniform color and without cleavage or fissures which shorten the life expectancy of prior art stones cut from naturally occurring novaculite deposits.
Of course it has previously been suggested in the prior art to artificially produce abrasive composite materials. Perhaps one of the earliest patents relating to this art is U.S. Pat. No. 1,983,082 issued to W. L. Howe on Dec. 4, 1934. The latter referece describes how raw materials of granular abrasive material may be formulated with a vitrified bond after a mixing process. The bond is matured after pressing and shaping by heating until vitrification occurs, at which point cementing of the various constituent grain occurs. Such a bond results in regions of voids, between which the "grinding" abrasive characteristics of the resultant stone are produced.
However, no process known to us has hitherto yielded artificial novaculite exhibiting superior qualities of homogeniety, uniform density, high strength (both tensile and abrasive), high MOH hardness in excess of MOH 7.0, and virtually complete omission of fissures, contaminating incursions and the like.